Fokker-Planck equation with variable diffusion coefficient in the Stratonovich approach

We consider the Langevin equation with multiplicative noise term which depends on time and space. The corresponding Fokker-Planck equation in Stratonovich approach is investigated. Its formal solution is obtained for an arbitrary multiplicative noise term given by $g(x,t)=D(x)T(t)$, and the behaviors of probability distributions, for some specific functions of $D(x)$% , are analyzed. In particular, for $D(x)\sim | x| ^{-\theta /2}$, the physical solutions for the probability distribution in the Ito, Stratonovich and postpoint discretization approaches can be obtained and analyzed.Comment: 6 pages in LATEX cod

Gate-bias assisted charge injection in organic field-effect transistors

The charge injection barriers in organic field-effect transistors (OFETs) seem to be far less critical as compared to organic light-emitting diodes (OLEDs). Counter intuitively; we show that the origin is image-force lowering of the barrier due to the gate bias at the source contact; although the corresponding gate field is perpendicular to the channel current. In coplanar OFETs; injection barriers up to 1 eV can be surmounted by increasing the gate bias; enabling extraction of bulk transport parameters in this regime. For staggered transistors; however; the injection is gate-assisted only until the gate bias is screened by the accumulation channel opposite to the source contact. The gate-assisted injection is supported by two-dimensional numerical charge transport simulations that reproduce the gate-bias dependence of the contact resistance and the typical S-shaped output curves as observed for OFETs with high injection barriers.

Low-voltage polymer field-effect transistors for nonvolatile memories

We demonstrate organic nonvolatile memories based on transistors, made from spin-coated polymers, that have programming voltages of 15 V and good data retention capabilities. The low-voltage programmable ferroelectric field-effect transistors were obtained by an optimized ferroelectric polymer deposition method using cyclohexanone as a solvent from which films can be obtained that are thin, smooth and defect free. The data retention characteristics were measured for 3 h under constant read conditions. Extrapolation predicts that the data retention capability exceeds 10 years. (C) 2005 American Institute of Physics

Electron Trap Dynamics in Polymer Light-Emitting Diodes

Semiconducting polymers are being studied intensively for optoelectronic device applications, including solution-processed light-emitting diodes (PLEDs). Charge traps in polymers limit the charge transport and thus the PLED efficiency. It is firmly established that electron transport is hindered by the presence of the universal electron trap density, whereas hole trap formation governs the long-term degradation of PLEDs. Here, the response of PLEDs to electrical driving and breaks covering the timescale from microseconds to (a few) hours is studied, thus focusing on electron traps. As reference polymer, a phenyl-substituted poly(para-phenylene vinylene) (PPV) copolymer termed super yellow (SY) is used. Three different traps with depths between approximate to 0.4 and 0.7 eV, and a total trap site density of approximate to 2 x 10(17) cm(-3) are identified. Surprisingly, filling of deep traps takes minutes to hours, at odds with the common notion that charge trapping is complete after a few hundred microseconds. The slow trap filling feature for PLEDs is confirmed using poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and poly(3-hexylthiophene) (P3HT) as active materials. This unusual phenomenon is explained with trap deactivation upon detrapping and slow trap reactivation. The results provide useful insight to pinpoint the chemical nature of the universal electron traps in semiconducting polymers

Enhanced dissociation of charge-transfer states in narrow band gap polymer:fullerene solar cells processed with 1,8-octanedithiol

The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processed from solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of the experimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene, these cells do not produce a recombination-limited photocurrent. Modeling of the experimental data reveals that a sixfold reduction in the decay rate of photogenerated bound electron–hole pairs can account for the marked increase in short-circuit current density and fill factor. At short-circuit conditions, the dissociation probability of bound pairs is found to increase from 48% to 70%.

Manipulation of charge carrier injection into organic field-effect transistors by self-assembled monolayers of alkanethiols

Charge carrier injection into two semiconducting polymers is investigated in field-effect transistors using gold source and drain electrodes that are modified by self-assembled monolayers of alkanethiols and perfluorinated alkanethiols. The presence of an interfacial dipole associated with the molecular monolayer at the metal/semiconductor interface changes the work function of the electrodes, and, hence, the injection of the charge carriers. The FET characteristics are analysed with the transfer line method and the hole injection into poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and regio-regular poly(3-hexyl)thiophene (rr-P3HT) is investigated. The device parameters are corrected for the contact resistances of the electrodes and the mobilities of the polymers (MEH-PPV, µFET = 4 × 10-4 cm2 V-1 s-1 and rr-P3HT, µFET = (1–2) × 10-2 cm2 V-1 s-1) are determined. The contact resistance obtained for the SAM-modified electrodes is at least one order of magnitude larger than for untreated contacts.

Diffuse Gamma-Ray Emission from Starburst Galaxies and M31

We present a search for high energy gamma-ray emission from 9 nearby starburst galaxies and M31 with the EGRET instrument aboard CGRO. Though the diffuse gamma-ray emission from starburst galaxies was suspected to be detectable, we find no emission from NGC 253, M82 nor from the average of all 9 galaxies. The 2 sigma upper limit for the EGRET flux above 100 MeV for the averaged survey observations is 1.8 x 10-8 ph cm-2 s-1. From a model of the expected radio and gamma-ray emission, we find that the magnetic field in the nuclei of these galaxies is > 25 micro Gauss, and the ratio of proton and electron densities is < 400. The EGRET limits indicate that the rate of massive star formation in the survey galaxies is only about an order of magnitude higher than in the Milky Way. The upper limit to the gamma-ray flux above 100 MeV for M31 is 1.6 x 10-8 ph cm-2 s-1. At the distance of M31, the Milky Way flux would be over twice this value, indicating higher gamma-ray emissivities in our Galaxy. Therefore, since the supernova rate of the Milky Way is higher than in M31, our null detection of M31 supports the theory of the supernova origin of cosmic rays in galaxies.Comment: 17 pages, plus 1 Postscript figure, AAS Latex macros v4.0, accepted for publication in ApJ Main Journa

Does the Blazar Gamma-Ray Spectrum Harden with Increasing Flux? Analysis of 9 Years of EGRET Data

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO) discovered gamma-ray emission from more than 67 blazars during its 9 yr lifetime. We conducted an exhaustive search of the EGRET archives and selected all the blazars that were observed multiple times and were bright enough to enable a spectral analysis using standard power-law models. The sample consists of 18 flat-spectrum radio quasars(FSRQs), 6 low-frequency peaked BL Lac objects (LBLs) and 2 high-frequency peaked BL Lac objects (HBLs). We do not detect any clear pattern in the variation of spectral index with flux. Some of the blazars do not show any statistical evidence for spectral variability. The spectrum hardens with increasing flux in a few cases. There is also evidence for a flux-hardness anticorrelation at low fluxes in five blazars. The well-observed blazars (3C 279, 3C 273, PKS 0528+134, PKS 1622-297 PKS 0208-512) do not show any overall trend in the long-term spectral dependence on flux, but the sample shows a mixture of hard and soft states. We observed a previously unreported spectral hysteresis at weekly timescales in all three FSRQs for which data from flares lasting for ~(3-4) weeks were available. All three sources show a counterclockwise rotation, despite the widely different flux profiles. We analyze the observed spectral behavior in the context of various inverse Compton mechanisms believed to be responsible for emission in the EGRET energy range. Our analysis uses the EGRET skymaps that were regenerated to include the changes in performance during the mission

Unification of trap-limited electron transport in semiconducting polymers

Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking. Here we investigate electron transport in a wide range of semiconducting polymers by current-voltage measurements of single-carrier devices. We observe for this materials class that electron transport is limited by traps that exhibit a Gaussian energy distribution in the bandgap. Remarkably, the electron-trap distribution is identical for all polymers considered: the number of traps amounts to 3 × 1023 traps per m3 centred at an energy of ∼3.6 eV below the vacuum level, with a typical distribution width of ∼0.1 eV. This indicates that the electron traps have a common origin that, we suggest, is most likely related to hydrated oxygen complexes. A consequence of this finding is that the trap-limited electron current can be predicted for any polymer. © 2012 Macmillan Publishers Limited. All rights reserved

Negative length orbits in normal-superconductor billiard systems

The Path-Length Spectra of mesoscopic systems including diffractive scatterers and connected to superconductor is studied theoretically. We show that the spectra differs fundamentally from that of normal systems due to the presence of Andreev reflection. It is shown that negative path-lengths should arise in the spectra as opposed to normal system. To highlight this effect we carried out both quantum mechanical and semiclassical calculations for the simplest possible diffractive scatterer. The most pronounced peaks in the Path-Length Spectra of the reflection amplitude are identified by the routes that the electron and/or hole travels.Comment: 4 pages, 4 figures include